When you want to save a version of a Workflow either save or publish the workflow.
From the Workflow designer export the workflow and save it to a location and name the file appropriately
When you need to revert to the older version of the Workflow open the Nintex Workflow designer and import the Workflow.
Versioning for Nintex forms for O365 is automatically switched on.
To restore a Form navigate to https://<TenantName>/Site/NintexForms
Find the form that needs to be restored to a previous version and right click on it or click on the ellipse. Navigate to Version History. A dialog will appear with the previous versions. Select the appropriate version and this will now be the published form.